Optical recording medium has been one of the most successful high capacity data storage media in the market, especially the CD-recordable (CD-R). CD-R disk comprises a spirally pre-grooved polymeric substrate, a recording layer, a reflecting layer and a protecting layer.
An optical sensitive material, usually a dye, is used in the recording layer. The dye material is spin-coated on the spirally pre-grooved polymeric substrate. A gold or silver reflecting layer is sputtered on the recording layer. An UV-cured protecting lacquer is then spin-coated on the reflecting layer. When writing information into the CD-R disk, laser beam is guided along the pre-groove and energy is released in a definite short pulse time. The dye in the pre-groove absorbs the energy of the laser beam and goes through a chemical change, usually decomposition, due to the increase of temperature. The increase of temperature will not only make the dye to decompose but also cause the polymeric substrate to melt and deform. Thus an optical difference is created between area of the pre-groove exposed by laser beam and area not exposed by laser beam. The digital information is thus recorded. Upon reading, laser beam with less energy is used.
The storage capacity of the optical recording medium is largely determined by the wavelength of the laser beam. The laser beam used in CD-recordable (CD-R) has a wavelength of about 780 nm. Diameter of the laser stylus (laser spot) is about 1.04 .mu.m for a focussing lens with numerical aperture (NA) value of 0.45. The size of the disk and the diameter of the optical stylus of the laser thus determine the maximum storage capacity of the CD-R disk.
In order to increase the storage capacity of the optical medium, using a laser beam with shorter wavelength and a focussing lens with high NA value are the most effective ways. Digital Versatile Disk (DVD) is created by applying this concept. The recordable version of DVD is called DVD-R. Wavelength of the recording laser beam for DVD-R is 635 nm or 650 nm. Wave length of the reading laser beam for DVD-R is 650 nm with less power. Diameter of the optical stylus for 635 nm laser is about 0.635 .mu.m with a focussing lens of numerical aperture value of 0.6. Thus the track pitch between grooves can be decreased to 0.74 .mu.m and the storage capacity can be increased up to 4.7 GB for a single side DVD-R. If the storage capacity needs to be larger, then the numerical aperture has to be further increased.
The performance of DVD-R disc depends mostly on the properties of dye in the recording layer. These properties include optical properties, chemical properties, thermal properties, solubility, and solution properties. Selecting a proper dye with proper properties for the recording layer of DVD-R is very important.
Pentamethine cyanine dye has been well known for its successful application in CD-R. So, when searching for a suitable dye material for DVD-R, cyanine dye is one of the candidates to be considered. But due to the shorter laser beam wavelength used in writing and reading of DVD-R, trimethine cyanine is used instead of petamethine cyanine in order to get a suitable reflection and modulation of the DVD-R disk.
Among the different cyanine dyes, indolenine and benzindolenine-based cyanine dyes are most frequently applied in CD-R disk manufacturing and are also most frequently tried for DVD-R disk. U.S. Pat. No. 5,976,658 described the use of trimethine-based cyanine dye having symmetry or asymmetry indolenine structure for DVD-R application. JP 11-53761, 10-337959, 10-278426 also gave examples exclusively of applying trimethine based indolenine cyanine dyes in DVD-R Disk.
In JP 10-188339, examples were given about using symmetrical heterocyclic trimethine cyanine dye in high density optical recording media application.
There are still needs for finding better cyanine dyes for high density optical recording disk.
It is the first object of this invention to provide better timethine cyanine dyes with better recording properties for high density optical recording application.
It is the second object of this invention to provide an optical recording medium using the trimethine cyanine dye as its optical recording material.